Alkyl Halides and Nucleophilic Substitution

Vocabulary practice flashcards covering terminology, reaction types, kinetics, stereochemistry, and solvent effects for alkyl halides and nucleophilic substitution reactions.

Alkyl halide

An organic molecule containing a halogen atom ($X = \text{F}, \text{Cl}, \text{Br}, \text{I}$) bonded to an $sp^3$ hybridized carbon atom.

Primary ($1^\circ$), secondary ($2^\circ$), and tertiary ($3^\circ$) alkyl halides

The classification of alkyl halides based on the number of carbons bonded directly to the carbon bearing the halogen atom.

Vinyl halide

An organic halide in which a halogen atom is bonded directly to a $C=C$ double bond ($sp^2$ hybridized carbon), making it unreactive in Chapter 7 substitution and elimination reactions.

Aryl halide

An organic halide in which a halogen atom is bonded directly to an $sp^2$ hybridized carbon of a benzene ring.

Allylic halide

An organic halide having a halogen atom bonded to an $sp^3$ hybridized carbon atom that is adjacent to a $C=C$ double bond.

Benzylic halide

An organic halide having a halogen atom bonded to an $sp^3$ hybridized carbon atom that is adjacent to a benzene ring.

Bromomethane ($\text{CH}_3\text{Br}$)

A naturally occurring atmospheric alkyl halide formerly used as a soil fumigant, phased out in most developed countries because of its ozone-depleting properties.

Dichloromethane ($\text{CH}_2\text{Cl}_2$)

An important organic solvent that was historically used to decaffeinate coffee.

Halothane ($\text{CF}_3\text{CHClBr}$)

A safe general anesthetic developed as an alternative to hazardous compounds such as $\text{CHCl}_3$ and diethyl ether.

Elimination reaction (of alkyl halides)

A reaction in which an alkyl halide loses elements of $\text{HX}$ when treated with a Brønsted–Lowry base to form a new $\pi$ bond (an alkene).

Nucleophile

A Lewis base (neutral or negatively charged) possessing a lone pair or $\pi$ bond that attacks an electron-deficient atom, most commonly carbon.

Leaving group

An atom or group of atoms that departs with the electron pair of the heterolytically cleaved $\text{C}-\text{X}$ bond during a substitution or elimination reaction.

Basicity–leaving group relationship

The principle that weaker bases make better leaving groups because stable conjugate bases are better able to accept an electron pair.

Steric hindrance

A decrease in chemical reactivity resulting from the spatial crowding of bulky groups at the site of a reaction.

Nonnucleophilic base

A sterically hindered base (such as tert-butoxide) that remains strongly basic toward small protons but is a poor nucleophile toward crowded carbons.

Polar protic solvent

A polar solvent containing $\text{O}-\text{H}$ or $\text{N}-\text{H}$ bonds capable of intermolecular hydrogen bonding that solvates both cations and anions effectively.

Polar aprotic solvent

A polar solvent that exhibits dipole-dipole interactions but lacks $\text{O}-\text{H}$ or $\text{N}-\text{H}$ bonds, solvating cations well while leaving anions relatively unshielded ('naked') and reactive.

$\text{S}_\text{N}2$ mechanism

A bimolecular nucleophilic substitution reaction proceeding through a single concerted step, following second-order kinetics: $\text{rate} = k[\text{RX}][:\!\text{Nu}^-]$.

Backside attack

The mechanism of nucleophile approach from the side directly opposite the leaving group ($180^\circ$ away), characteristic of all $\text{S}_\text{N}2$ substitutions.

Inversion of configuration

The complete reversal of three-dimensional stereochemical arrangement at a stereocenter resulting from $\text{S}_\text{N}2$ backside attack.

$\text{S}_\text{N}1$ mechanism

A stepwise nucleophilic substitution proceeding through rate-determining heterolysis of the $\text{C}-\text{X}$ bond to form a carbocation intermediate, following first-order kinetics: $\text{rate} = k[\text{RX}]$.

Carbocation

An $sp^2$ hybridized, trigonal planar reactive intermediate having three groups around a positively charged carbon and a vacant $p$ orbital extending above and below the molecular plane.

Racemization

The formation of equal amounts of two enantiomeric products from an achiral planar intermediate in an $\text{S}_\text{N}1$ reaction due to unhindered attack from both faces.

Inductive effect (in carbocations)

The electronic stabilization of a carbocation's positive charge via the pull/donation of electron density through surrounding $\sigma$ bonds by alkyl groups.

Hyperconjugation

The stabilizing electronic interaction resulting from the overlap of an adjacent filled $\sigma$ bond with an empty $p$ orbital, delocalizing positive charge over a larger volume.

Organic synthesis

The systematic preparation of a target organic compound from readily available starting materials through one or more discrete reaction steps.